The invention relates to a device for optical distance measurement according to the preamble of the independent claim.
Optical distance-measuring devices as such have been known for some time and are now sold commercially in large numbers. These devices emit a modulated, respectively pulsed, light beam, which is directed toward the surface of a desired target object, whose distance from the device is to be ascertained. A portion of the returning light, which has been reflected or scattered by the target object, is detected by the device and used to ascertain the distance in question.
The application range of such distance-measuring devices generally comprises distances in the range of a few centimeters to several hundred meters. Depending upon the paths to be measured and the reflectance of the target object, different requirements result for the light source, the quality of the measurement beam as well as for the detector.
The optical distance-measuring devices known from the technical field can basically be divided up into two categories depending on the configuration of the transmission and reception channels necessarily present in the device.
On the one hand, there are devices with which the transmission channel is disposed a certain distance away from the reception channel so that the respective optical axes extend in parallel to each other but at a distance away from each other. On the other hand, there are monoaxial measuring devices with which the reception channel extends coaxially with the transmission channel.
The former biaxial measurement systems have the advantage that a complex beam-splitting system is not required to select the returning measurement signal, thereby also enabling, e.g., optical crosstalk from the transmission path directly into the reception path to be suppressed to a greater extent.
On the other hand, biaxial distance-measuring devices have among other things the disadvantage that detection problems may arise due to parallax when close-range distance measurements are performed. In this case, the image of the target object on the detector surface of the device—said image being unambiguously located on the detector even when target distances are great—moves increasingly further away from the optical axis of the reception path as the measurement distance decreases, and in addition the beam cross-section in the detector plane changes considerably. As a result, the measurement signal that is detected may approach zero in the close range of detection, i.e. when the distance between the target object and the measuring device is short, if no further measures are taken in the device.
Although measuring devices of this type can be optimized for a certain distance range, this requires, however, that the measuring range that is actually accessible to the measuring device be substantially limited.
The German patent publication DE 10 130 763 A1 makes a device known for optically measuring distance over a large measuring range, which includes a transmission unit with a light source for emitting modulated optical radiation toward a target object, and with which the receiving unit having an optical detector disposed in this measuring device, which serves to receive the optical radiation returning from the target object, is located on a reception axis, which is disposed at a distance away from the optical axis. The active, photosensitive surface of the detector of the receiving unit described in the German publication DE 10 130 763 A1 tapers in the direction of a beam displacement for decreasing target object distances that results due to a parallax of the returning measurement radiation.
The German patent publication DE 10 051 302 A1 makes known a laser distance-measuring device for close range and long-range that includes a special receiver with a transmission channel and a reception channel. The transmission channel consists of a transmission lens, in whose focal point a laser light source is disposed. The reception channel consists of a reception lens, in whose focal plane a receiver system is disposed. The optical axes of the transmission lens and the reception lens extend in parallel with each other with a finite spacing between them. The receiver system of the laser distance-measuring device described in the German publication DE 100 51 302 A1 is a photodiode chip system with at least two active photodiode surfaces disposed on a straight line, which intersects the optical axes of the transmission and reception lens of said device.
The German patent publication DE 10 2006 013292 A1 or also the German patent publication DE 100 51 302 A1 stipulates how an optimized reception behavior can be achieved with the aid of the geometric configuration of the active surface of a photodetector. The common aim of optimizing said reception behavior is thereby to simultaneously optimize three target parameters by distinguishing between a main detection surface for large distances and variably configured peripheral detection ranges for small distances:
On the one hand, the suitable configuration of the active photodetector surface ensures that a sufficiently strong measurement signal is available over the relevant distance range. This relates especially to the close range, wherein the centrally focused spot in the long range moves laterally relative to the spot position in the far field.
The second aim consists of configuring the photodetector surface such that the dynamics of the signal levels, which occur as a result of the optimal reception power with regard to shorter object distances increasing by the square of the reciprocal value of the distance, are to be evened out by the specific detector type. That means in the ideal case that the detector surface is configured such that a maximum signal level to be predetermined is not exceeded. The aim is thereby, for example, to avoid an overmodulation of the measurement amplifier.
Both optimizations are to be carried out under the constraint that the total available detector surface is to be held as small as possible so that as little as possible ambient light reaches the detector. The smaller the active surface is, the lower is the amplitude of the signal noise occurring as a result of ambient light. The detector surface thus forms the third target parameter to be optimized, i.e minimized.
The aim of the present invention is to ensure—based on a device for optically measuring distance according to the technical field—that firstly the most constant reception signal possible can be measured across the largest possible measuring range (i.e. a reception signal whose signal amplitude changes only slightly with the distance to the target object), that in so doing secondly the dependency on ambient light is minimized by an active detector surface, which is as small as possible and in addition thirdly the requirements for the adjustment precision of the optics of the device are held low.
This aim is met using a device according to the invention for optically measuring distance that includes the characteristics of the independent claim.